Java架构师笔记丨常见的错误 SQL 用法，你中招了吗？

去掉 exists 更改为 join，能够避免嵌套子查询，将执行时间从1.93秒降低为1毫秒。

新的执行计划：

+----+-------------+-------+--------+ -----+------------------------------------------+---------+ -----+------+ -----+

| id | select_type | table |type| possible_keys | key | key_len | ref | rows | Extra |

+----+-------------+-------+--------+ -----+------------------------------------------+---------+ -----+------+ -----+

| 1 | SIMPLE | m | ref | | idx_message_info | 122 | const | 1 | Using index condition |

| 1 | SIMPLE | n | eq_ref | | PRIMARY | 122 | ighbor_id | 1 | Usingwhere|

| 1 | SIMPLE | sra | ref | | idx_user_id | 123 | const | 1 | Usingwhere|

+----+-------------+-------+--------+ -----+------------------------------------------+---------+ -----+

6、条件下推

外部查询条件不能够下推到复杂的视图或子查询的情况有：

聚合子查询;

含有 LIMIT 的子查询;

UNION 或 UNION ALL 子查询;

输出字段中的子查询;

如下面的语句，从执行计划可以看出其条件作用于聚合子查询之后：

确定从语义上查询条件可以直接下推后，重写如下：

执行计划变为：

+----+-------------+-----------+------+---------------+-------+---------+-------+------+--------------------+

| id | select_type | table |type| possible_keys | key | key_len | ref | rows | Extra |

+----+-------------+-----------+------+---------------+-------+---------+-------+------+--------------------+

| 1 | SIMPLE | operation | ref | idx_4 | idx_4 | 514 | const | 1 | Usingwhere; Using index |

+----+-------------+-----------+------+---------------+-------+---------+-------+------+--------------------+

7、提前缩小范围

先上初始 SQL 语句：

该SQL语句原意是：先做一系列的左连接，然后排序取前15条记录。从执行计划也可以看出，最后一步估算排序记录数为90万，时间消耗为12秒。

+----+-------------+-------+--------+---------------+---------+---------+-----------------+--------+----------------------------------------------------+

| id | select_type | table |type| possible_keys | key | key_len | ref | rows | Extra |

+----+-------------+-------+--------+---------------+---------+---------+-----------------+--------+----------------------------------------------------+

| 1 | SIMPLE | o | ALL | NULL | NULL | NULL | NULL | 909119 | Usingwhere; Using temporary; Using filesort |

| 1 | SIMPLE | u | eq_ref | PRIMARY | PRIMARY | 4 | o.uid | 1 | NULL |

| 1 | SIMPLE | p | ALL | PRIMARY | NULL | NULL | NULL | 6 | Usingwhere; Using join buffer (Block Nested Loop) |

+----+-------------+-------+--------+---------------+---------+---------+-----------------+--------+----------------------------------------------------+

由于最后 WHERE 条件以及排序均针对最左主表，因此可以先对 my_order 排序提前缩小数据量再做左连接。SQL 重写后如下，执行时间缩小为1毫秒左右。

再检查执行计划：子查询物化后(select_type=DERIVED)参与 JOIN。虽然估算行扫描仍然为90万，但是利用了索引以及 LIMIT 子句后，实际执行时间变得很小。复制代码

+----+-------------+------------+--------+---------------+---------+---------+-------+--------+----------------------------------------------------+

| id | select_type | table |type| possible_keys | key | key_len | ref | rows | Extra |

+----+-------------+------------+--------+---------------+---------+---------+-------+--------+----------------------------------------------------+

| 1 | PRIMARY | | ALL | NULL | NULL | NULL | NULL | 15 | Using temporary; Using filesort |

| 1 | PRIMARY | u | eq_ref | PRIMARY | PRIMARY | 4 | o.uid | 1 | NULL |

| 1 | PRIMARY | p | ALL | PRIMARY | NULL | NULL | NULL | 6 | Usingwhere; Using join buffer (Block Nested Loop) |

| 2 | DERIVED | o | index | NULL | idx_1 | 5 | NULL | 909112 | Usingwhere|

+----+-------------+------------+--------+---------------+---------+---------+-------+--------+----------------------------------------------------+

8、中间结果集下推

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